Electron gun for beam-type tubes



D. H. PREIST ELECTRON GUN FOR BEAM-TYPE TUBES July 8, 1958 Filed Pet. 5, 1953 IN V EN TOR.

Donald H. Pref 7" A T TOQNE Y ire ELECTRUN GUN FUR BEAM-TYPE TUBES? Application tictoher 5, I953, genial No. 384,097

9 Claims. (Cl. 315-3) My invention relates to an improved electron gun structure for beam-type tubes such as klystrons, traveling wave tubes, and the like.

The principal object of my invention is to provide a simple gun structure for producing a high perveance electron beam.

Another object is to provide an electron gun wherein the perveance of the beam may be varied externally of the tube.

Another object is to provide a gun structure which protects the cathode from positive ion bombardment.

A further object is to provide an electron gun which enables the beam to be modulated in an improved manner.

The invention possesses other objects and features of advantage, some of which, with the foregoing, will be set forth in the following description of my invention. .It is to be understood that I do not limit myself to this disclosure of species of my invention, as I may adopt variant embodiments thereof within the scope of the claims.

Referring to the drawing:

The single figure is a vertical sectional view, partly diagrammatic, showing a beam-type tube embodying the improvements of my invention.

It is often advantageous in lrlystrons, traveling wave tubes and similar beam-type devices to use a high perveance beam, namely, a beam in which the ratio of the current is the beam to the electron velocity is as high as possible. With electron guns constructed in accord ance with conventional practice where the electrons are accelerated by a positive potential on the anode with respect to the cathode, the highest value of perveance obtainable is generally considered to be about x10- (amperes per volt Expressed in rnicroamperes this is called a perveance 10 gun. Such high perveances are very diflicult to attain because of beam focusing problems and the very close tolerances required on the dimensions of the gun structure. As a result, tubes in actual practice have relatively low perveance guns, say of the order of 1X10 (amperes per volt Such perveance 1 guns are not too difficult to make on a quantity production basis and for that reason are frequently used in commercial tubes, even though higher perveance beams would be desired.

With my improvements the perveance of an electron gun may be increased manyfold without difiiculty. As shown in the drawing, the structure comprises a diskshaped cathode 2 supported by a stem 3 and heated by a suitable filament or heater 4. The electrons from the cathode are focused into a circular beam 6 through an apertured anode 7 by a suitable tubular focusing electrode 3, the anode 7 being operated at a positive potential with respect to the cathode as indicated by the D. C. voltage source V connected between the anode and cathode. An insulating cylinder 9 supports the cathode structure in vacuum tight relationship with respect to the stem @ ice 6m anode. The above described construction is conventional practice, the details of such electron gun design being well understood by those skilled in the art. As previously mentioned, it is not top difiicult to produce a beam having a perveance value of unity with guns of this general type.

In my improved construction additional means are provided as a part of the gun structure and ahead of the anode 7 for decelerating the electrons after they emerge from the anode. Such means preferably comprises a decelerating electrode Ill spaced from the anode 7 by an insulating cylinder 12 which also forms part of the evacuated envelope of the tube. Electrode ll is apertured to receive the beam 6 and preferably has a tubular projection 13 opposite a similar projection 14 on the anode 7 to provide a gap 16. Electrode i1 is also operated at a positive potential with respect to the cathode as indicated by the D. C. voltage source V but the decelerating electrode voltage V is made less than the anode voltage V so that the electrons are decelerated as they pass through gap 316.

The beam emerging from the decelerating electrode 11 of my improved gun structure then passes into a main radio frequency section of the tube indicated by the dotted line 17, which latter section may be a ldystron type of device having resonant cavities or a traveling wave tube or any other similar beam-type of device adapted for interaction with the beam.

The principle or" operation of my improved eiectron gun is as follows:

First considering the initial gun formed by the cathode 2 and anode '7, if I is the beam current and K is the perveance of such cathode-anode structure in microamperes per volt the following relationship exists:

Now considering the addition of the decelerating device it is seen that with a properly focused beam the current I of the beam emerging from the electrode 11 is the same as that issuing from the anode '7, but the velocity of the electrons in the emerging beam will be reduced by reason of the lower voltage V on the decelerating electrode It. This is expressed as:

rt I i VB X or as 3/2 1 1 [2] xv s/z from which the efiective perveance of the combination of the initial gun and the decelerating device is given In other words, the efiective perveance of the final combination is a function of the voltage ratio For example, if

and K=1, the eflective )erveance of my improved gun structure will be l 5 =ll.l5. Theoretically there appears to be no reason way the ratio should not be increased indefinitely.

Thus I have provided a simple means for producing 3 high perveance beams with an initial low perveance cathode-anode structure which is relatively easy to manufacture in mass production operations. in addition to increasing beam perveance, my improved gun structure has further advantages.

Heretofore beam perveance has been a fixed parameter set by the geometry of the gun. With my improved electron gun having a decelerating electrode, the beam perveance may be controlled or changed externally of the tube by adjusting the voltage ratio VA VB Also because the anode 7 may be operated at a potential more positive than the electrodes 2 5.1, any positive ions in the tube structure ahead of the gap 6 will not be attracted back toward the cathode 2. The cathode is thus protected against positive ion bombardment which is frequently destructive in beam-type tubes, particularly when oxide-coated cathodes are used in conjunction with high accelerating fields.

Another advantage is that it is possible to modulate the beam current 100% by varying the voltage V while leaving the voltage V fixed. This is particularly adapted for modulation by very fast wave forms such as short pulses and enables low power modulators to be used for modulating relatively high power beams.

It is thus seen that my improved construction has important advantages other than that of increasing the beam perveance. In any case the advantages came about because of the particular combination of elements making up the gun structure, namely, a pair of spaced electrodes 7 and 11 having apertures coaxially aligned with the cathode 2 and disposed between the latter and the main section 17 of the tube, together with means such as the focusing device 8 for focusing the beam 6 of electrons through the apertures of the spaced electrodes '7 and ill, which electrodes are provided wtih terminals externally of the tube envelope.

I claim:

1. A beam-type tube having a radio frequency section and a controllable pcrveance gun structure comprising a cathode for initiating an electron beam, and an apertured anode having a tubular projection thereon and positioned intermediate said cathode and said radio frequency section, said anode being adapted to operate at a certain D. C. potential with respect to said cathode to produce a certain beam perveance and means inclurf. d in said gun structure for controlling said perveance including an apertured electrode intermediate said anode and said radio frequency section and having a tubular projection extending in the direction of the tubular pro jection on said anode, said last-mentioned means further including means for operating said electrode at a D. C. potential lower than said certain D. C. potential to increase said beam perveance.

2. In an electron tube of the beam type ha ing main radio frequency section, an electron gun comprising cathode means for emitting a beam of electrons, anode means intermediate said cathode means and said section for accelerating the electrons and means including an electrode intermediate said anode means and said section for increasing the efiective perveance of said tube, said lastmentioned means including means for controlling said beam of electrons in accordance with the formula:

K 1 v3 ri where I is the beam current in micrc-airiperes, V is the anode voltage in volts, V is the .ctrode voltage in volts, and K is the perveance Ll""" ble perea ce comprising a cathode, a first insulating cylinder connected to said cathode, an apertured anode abutting said cylinder and axially aligned with said cathode, a second insulating cylinder connected to said anode and an apcrtured electrode abutting said second cylinder and having a tubular portion extending toward said anode, said electrode being axially aligned with said anode.

4. An electron gun in accordance with claim 3 wherein said anode has a tubular portion extending toward the tubular portion of said electrode.

5. A variable perveance electron gun for developing a stream of electrons in a velocity modulated tube, said gun comprising a cathode having a concave emissive surface, a focusing electrode adjacent said cathode, a first anode mounted in front of said concave emissive surface of said cathode and insulated therefrom, said first anode having an aperture therethrough coaxial with said emissive surface of said cathode, and a second anode mounted on the opposite side of said first anode from said cathode, said second anode being insulated from said first anode and said cathode and having an aperture therethrough coaxial with the aperture through said first anode, said first anode electrostatically shielding said cathode and said focusing electrode from said second anode.

6. A variable perveance electron gun for developing a stream of electrons in a velocity modulated tube, said gun comprising a cathode disk having a concave emissive surface, a tubular focusing electrode surrounding said cathode disk, 21 first insulating cylinder coaxial with said cathode and said focusing electrode, said cathode and said focusing electrode being suitably mounted on a stem member which is hermetically sealed to and closes one end of said first insulating cylinder, said first anode disk having an aperture therein coaxial with said cathode and said focusing electrode, a second insulating cylinder co axial with said anode disk aperture, one end of said second insulating cylinder being hermetically sealed to the opposite side of said first anode disk from said first insulating cylinder, and a second anode disk hermetically sealed to the other end of said second insulating cylinder, said second anode disk having an aperture therethrough coaxial with the aperture in said first anode disk.

7. A variable perveance electron gun for a velocity modulated tube, said gun comprising a cathode disk having a concave emissive surface, a tubular focusing electrode coaxially surrounding said cathode disk, a first insulating cylinder coaxial with and surrounding said cathode and said focusing electrode, said cathode and said focusing electrode being suitably mounted on a stem member which is hermetically sealed across one end of said first insulating cylinder, a first anode disk hermetically sealed across said other end of said first insulating cylinder, said first anode disk having an aperture therein coaxial with said emissive surface of said cathode and said focus electrode, said first anode disk having an annular projection on the opposite side thereof from said cathode and surrounding said aperture whereby a passage way is provided having a length greater than its diameter, a second insulating cylinder coaxial with said aperture in said first anode disk, one end of said second insulating cylinder being hermetically sealed to the opposite side of said first anode disk from said first insulating cylinder, and a second anode disk hermetically sealed to the other end of said second insulting cylinder, said second anode disk having an aperture therethrough coaxially aligned with the aperture and passageway of the said first anode disk.

8. An electron gun for a velocity modulated tube, said gun comprising a cathode disk having a concave emissive surface, a tubular focusing electrode surrounding said cathode disk, a first ceramic insulating cylinder coaxial with and surrounding said cathode and said focusing electrode, said cathode and said focusing electrode being mounted on a stem member which is hermetically sealed to one end of said first insulating cylinder, a first anode disk hermetically sealed across the other end of said insulating cylinder, said first anode disk having an aperture therein coaxial with said emissive surface of said cathode, means including a voltage source for operating said first anode disk at a given d. c. potential with respect to said cathode to produce a given beam current, said emissive surface of said cathode, said focusing electrode, said first anode disk, and the aperture through said first anode disk being dimensioned and spaced from each other so that the diameter of the electron beam at said first anode disk is less than the diameter of said aperture in said first anode disk at said given current and potential, an annular projection on the opposite side of said first anode disk from said cathode and surrounding said aperture whereby a passageway is provided having a length greater than the diameter thereof, a second ceramic insulating cylinder coaxial with said aperture in said first anode disk, one end of said second insulating cylinder being hermetically sealed to the opposite side of said first anode disk from said first insulating cylinder, a second anode disk hermetically sealed to the other end of said second insulating cylinder, said second anode disk having an aperture coaxially aligned with the aperture and passageway of said first anode disk, and means including a voltage source for operating said second anode disk at a different potential from said first anode disk, said second anode disk being spaced from said first anode disk and said anode disk aperture in said second anode disk being dimensioned such that the diameter of said electron beam at said second anode disk is less than the diameter of said aperture in said first anode disk.

9. A velocity modulated electron tube including an electron gun, said gun comprising a cathode disk having a concave emissive surface, a tubular focusing electrode surrounding said cathode disk, said cathode and said focusing electrode being suitably mounted on a stem member, a first insulating cylinder coaxial with and surrounding said cathode and said focusing electrode, one end of said insulating cylinder being hermetically sealed to said stem member, the other end of said first insulating cylinder projecting beyond the end of said focusing electrode, a first anode disk hermetically sealed to the other end of said insulating cylinder, said first anode disk having an aperture therein coaxial with said cathode and said focusing electrode, means including a voltage source for operating said first anode disk at a given positive potential with respect to said cathode to produce a given beam current, said emissive surface of said cathode, said focusing electrode, said first anode disk, and said aperture in said first anode disk being dimensioned and spaced such that the electron beam produced thereby has a certain desired perveance, an annular projection on the opposite side of said anode dial; from cathode and surrounding said aperture whereby a passageway is provided having a length greater than the diameter thereof, a second insulating cylinder coaxial with said cathode, focusing electrode and aperture in said first anode disk, one end of said second insulating cylinder being hermetically sealed to the opposite sit e of said first anode disk from said first insulating cylinder, a second anode disk hermetically sealed to the other end of said second insulating cylinder, said second anode disk having an aperture therethrough coaxially aligned with the aperture and passageway of said first anode disk, and means including a voltage source for operating said second anode disk at a lower positive potential with respect to said first anode disk than said given potential to increase said certain perveance.

References Cited in the file of this patent UNITED STATES PATENTS 2,484,643 Peterson Oct. 11, 1949 2,517,726 Skellett Aug. 8, 1950 2,527,600 Touraton et al Oct. 31, 1950 2,581,243 Dodds Jan. 1, 1952 2,632,130 Hull Mar. 17, 1953 2,636,948 Pierce Apr. 28, 1953 2,762,948 Field Sept. 11, 1956 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 2,842,703 July 8, 1958 Donald H4, Pre'ist It is hereby certified that error appears in the-printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 1, line 42,, for "current is" read current in column 2, line 66 for "way read why golumn ,4, line 63, for "insulting" read insulating Signed and sealed this 23rd day of September 1958 SSEAL) ttest:

Attesting Officer ROBERT C. WATSON Commissioner of Patents 

